1. Field of the Invention
The present invention relates to a magnetic recording medium having a back coating layer formed on the back surface of a substrate opposite to a magnetic layer on the front surface of the substrate. More particularly, it relates to a magnetic recording medium having a back coating layer on the back surface of the substrate which contains a specific polyfluoropolyether and optionally a fluorine-containing solid lubricant.
2. Description of the Prior Art
Since a magnetic recording medium such as a magnetic tape comprises a substrate having smooth surfaces and a magnetic layer also has a smooth surface, it tends to closely contact guide rollers and the like during traveling and sticks to them. In case of a magnetic recording medium comprising a magnetic layer made by vacuum deposition of ferromagnetic metal or alloy on the substrate, since surface smoothness of the substrate is directly reflected on surface smoothness of the ferromagnetic metal thin layer and has a great influence on the electromagnetic conversion characteristics of the magnetic recording medium, an extremely smooth substrate should be used. For this reason, the substrate has a large coefficient of friction so that it easily sticks to the guide rollers, which results in elongation of the magnetic recording medium, stick slip or deterioration of the jitter characteristics.
To overcome these drawbacks, it has been proposed to provide a back coating layer containing a lubricant such as perfluoropolyether on a back surface of the substrate.
However, such a conventional back coating layer cannot sufficiently reduce the coefficient of friction of the substrate and in turn cannot improve the jitter characteristics and durability.